The invention relates to gas lasers and, in particular, to aerodynamic windows for optically transmitting high energy laser beam into the surrounding environment.
Aerodynamic windows are rather well known and have been discussed in a number of publications including, for example, U.S. Pat. Nos. 3,617,928, 3,907,409, 3,918,800, 3,973,321 and others. Generally considered, these windows are formed by a window gas driven at a high, jet-stream velocity across the laser beam and, of course, the jet streams serve the same functions as the more conventional solid windows. In other words, they contain the gas in the laser, maintain a desired laser cavity pressure and they optically transmit the laser beam into the outside environment.
As compared with the solid-type windows, aerodynamic windows are particularly suited for transmitting high energy, relatively large-diameter laser beams. Solid windows, as is known, absorb a great deal of beam energy and become difficult if not impossible to cool. Gas windows avoid this particular difficulty.
Several problems, however, do arise. For example, the use of jet streams, whether driven at subsonic or supersonic velocities, tend to produce optical distortions. For one reason, the gas which forms the window usually differs as a media from the interior gas of the laser and from the gas or air in the outside environment. The differing indices of refraction produce distortions in the beam path. Also, since the high-velocity, jet streams of the windows have a shear interface with the interior gas of the laser on one side and another such interface with the air on the outside further distortions result. Obviously, any such distortions reduce the optical quality of the window and, in particular, result in a significant loss of the far field intensity of the laser beam. To minimize this loss, the index of refraction gradients in the various media disposed in the beam path should be aligned as nearly as possible parallel to the propagation direction of the laser beam. Achievement of this parallel relationship, however, is not easily achieved especially when the window jets are driven at very high, supersonic velocities. In some laser applications high supersonic velocities are needed to maintain large interior and exterior pressure differentials. Lower subsonic velocities may be more desirable but, to some extent, their use is limited to applications having small pressure differentials. The window of the present invention is adapted primarily for small pressure differential lasers. More specifically, the present window preferably contemplates pressure differences on the order of 10 cm H.sub.2 O.
Another problem with aerodynamic windows is one of minimizing the consumption of the gas that forms the window. As will be understood, this window gas usually is directed by a nozzle radially across the laser beam where it is received by a diffuser conduit and conducted to an exhaust manifold. Window gas consumption consequently can be high enough to constitute a significant economic factor especially when the laser beam is large in diameter, i.e. on the order of 1 m or more. In particular, consumption will vary with the square of the span of the beam. Again, the present window contemplates use with large diameter, parallel (unfocused) laser beams so that the degree of window gas consumption is a signficant factor. The prior art has recognized these particular problems but, as far as presently is known, has not provided adequate solutions especially when the application involves the transmission with good optical quality of a high energy, large diameter laser beam.
It is therefore a primary object of the present invention to significantly improve the optical transmission quality of aerodynamic windows.
A further object is to provide a window capable of minimizing window gas consumption.
As indicated, the present invention primarily is concerned with high energy, large diameter laser beams and with lasers of the type having relatively small pressure differences between their interior cavity and the outside environment. In this latter regard, a further object is to maintain this pressure differential in a manner which itself is advantageous in minimizing window gas consumption.
These and other objects are achieved by utilizing an aerodynamic window formed of a pair of window-forming jets one of which has a shear interface with the interior gas of the laser while the other has a similar interface with the outside air. To minimize optical distortion, the jet stream interfaced with the laser gas has its index of refraction matched to that of the interior laser gas. The other jet similarly is matched with the refraction index of air. Also, the jets are driven at equalized velocities to minimize distortion at their mutual or boundary interface. A further refinement used to maintain the interior pressure of the laser cavity is to permit the contiguous pair of jet streams to bend a controlled amount responsively to the pressure differential. The degree of the bend, as well as the jet velocity, is controlled to achieve a minimum window gas consumption. Another preferred feature directed also at window gas consumption is provided by geometrically disposing the jet forming nozzles in the center of an annular laser beam so that the jet stream flows radially outwardly across the 360 degrees of the annular beam. This geometry reduces the beam span required for window effectiveness and correspondingly reduces window gas consumption.